You need to format ALL values in the column that way. It doesn't do anything to set one a certain way and expect them all to be treated as such. Either the column contains all numbers or the column contains all strings. As mentioned, the best default I've found is to force everything in the column to strings by prefacing everywhere where necessary with the (') character.
And, while it's not random, it is rather dependant upon what the first cell value is that you read in the driver or that the driver encounters. So, sometimes it will work well, sometimes it won't, all depending upon how you're operating with the excel file and such.
BTW, this is one reason why I avoid Excel for use in DDT drivers because it's rather unpredictable. CSV's while they can have the same problem are much easier to correct by just putting a schema.ini file in the directory with the CSV file where you indicate the column format. Then you don't have to worry about formatting the data itself, the schema.ini takes care of all of that for you.
